1. Field of the Invention
The present invention relates to an electrophotographic photoconductor which has high durability and realizes high definition. It further relates to an electrophotographic method using these photoconductors, an electrophotographic apparatus, and an electrophotographic process cartridge.
2. Description of the Related Art
In recent years, there has been a remarkable growth of information processing systems using electrophotography techniques. In particular, laser printers and digital copiers that change information into digital signals to record information by light have made remarkable improvements to print quality and reliability. In combination with high speed technology, they are now being applied to laser printers or digital copiers which can print in full color. As a result, it has now become important to obtain the dual objective of high definition and high durability as a function of required photoconductor.
In general, photoconductors used in electrophotographic laser printers and digital copiers employ organic photoconducting materials from the viewpoint of cost, productivity and non-pollution. Examples of organic photoconductors known in the art are photoconducting resins such as polyvinyl carbazole (PVK), charge transfer complex type such as PVK-TNF (2,4,7-trinitrofluoenone), pigment dispersion type such as phthalocyanine binders, and the discrete function type which combine a charge generating material with a charge transport material.
The mechanism of latent electrostatic-image formation in the discrete function type of photoconductor is as follows. The photoconductor is charged and irradiated with light, the light passes through a charge transport layer, and is absorbed by a charge generating material in the charge generating layer to generate a charge. The charge thus generated is implanted into the charge transport layer at the interface of the charge generating layer and charge transport layer, moves through the charge transport layer due to the electric field, and forms the latent electrostatic image by neutralizing the surface charge on the photoconductor.
However, when the organic photoconductor was used repeatedly, film scraping tended to occur, and if film scraping of the photoconducting layer was severe, the charging potential of the photoconductor decreased, photosensitivity deteriorated, the toner deposited due to scratches on the photoconductor surface, image density decreased or image quality seriously deteriorated, and the wear resistance of the photoconductor was consistently a major problem. In recent years, with higher speeds of electrophotography apparatus or smaller diameter photoconductors as devices become more compact, high durability of the photoconductor has become a much more important topic.
To achieve high durability of the photoconductor, a protective layer is usually provided on the outermost surface of the photoconductor, and this protective layer is given lubricant properties or hardened, or a filler is incorporated in the layer. The addition of a filler to the protective layer is a particularly effective way of improving the durability of the photoconductor. However, if the filler has strong electrical insulation properties, its resistance increases, and there is a considerable increase of residual potential. This residual potential rise is largely due to increase of resistance and an increase of charge trap sites which come about when the filler is incorporated. If a conductive filler is used, the resistance falls and the effect of residual potential increase is comparatively small, but then the image outline fades, image blurring occurs and there is a significant effect on image quality.
Therefore, in the related art, as it was difficult to use a filler with highly insulating properties, a filler with weaker insulating properties which had relatively little effect on residual potential was used, and a drum heater to heat the photoconductor was provided to deal with the image blurring produced. The heating of the photoconductor suppressed image blurring, however the provision of the drum heater necessitated an increase in the photoconductor diameter. This technique can therefore not be applied to the small diameter photoconductors which are now becoming common as electrophotographic equipment becomes more compact, and it is becoming difficult to achieve high durability with small diameter photoconductors. Also, if a drum heater is provided, the apparatus becomes more bulky and power consumption increases by a considerable extent, moreover a long time is required when starting up the apparatus, so many problems still remained to be solved.
If a filler with high electrical resistance is used, the increase of residual potential which is commonly observed leads to an increase of potential in the illuminated parts of the electrophotographic apparatus, which causes a decrease of image density and gradation. In order to compensate for this, it is necessary to increase the potential of the dark parts of the apparatus, but if the potential of the dark parts is increased, the electric field intensity increases, image defects such as toner background deposition occur, and the life of the photoconductor is also shortened.
As a means of suppressing residual potential rise in the related art, a method of using the protective layer as the photoconductive layer has been disclosed (Japanese Patent Application Publication (JP-B) No. 44-834, JP-B No. 43-16198, JP-B No. 49-10258). However, the light amount reaching the photoconductive layer decreased due to the absorption of light by the protective layer, there was a decrease in the sensitivity of the photoconductor, and its effect was only slight.
In another method, the average particle diameter of a metal or metal oxide contained as the filler is made equal to 0.3 μm or less (Japanese Patent Application Laid-Open (JP-A) No. 57-30846), so that the protective layer is effectively transparent, and accumulation of residual potential is suppressed. This method does have an effect in suppressing the increase of residual potential, but its effect is insufficient, and it still has not yet resolved the above problems. This is because the increase of residual potential when the filler is included, is probably due to charge traps or filler dispersibility if the filler is present, rather than to charge generating efficiency. Even if the average particle diameter of the filler is more than 0.3 μm, transparency can be obtained by increasing dispersibility, and even if the average particle diameter is less than 0.3 μm, the transparency of the film will decrease if the filler has a high degree of cohesion.
According to another method, a charge transport material is contained together with the filler in the protective layer (JP-A No. 04-281461), which increases the mechanical strength and suppresses residual potential rise. The addition of the charge transport material to the protective layer has the effect of improving the mobility of the charge, and is an effective way of reducing residual potential. However, if the considerable increase of residual potential resulting from the inclusion of the filler is due to increase of resistance and increase of trap sites when the filler is present, there will be a limit to the suppression of residual potential rise obtained by improving charge mobility. Therefore, the film thickness of the protective layer and the filler content must be decreased, and the necessary durability cannot be achieved.
There are other methods of suppressing residual potential rise, for example the addition of a Lewis acid to the protective layer (JP-A No. 53-133444), the addition of an organic protonic acid to the protective layer (JP-A No. 55-157748), the inclusion of an electron-accepting material (JP-A No. 02-4275), and the inclusion of a wax having an acid value of 5 (mg/KOH/g) or less (JP-A No. 2000-66434). These methods are thought to suppress the residual potential rise by improving the implantation of charge at the protective layer/electron transport layer interface, and making it easy for charge to reach the surface by forming a low resistance part in the protective layer. This method is found to have the effect of increasing residual potential, but it does tend to cause image blurring, and the superfluous effect on the image is obvious. Further, if an organic acid is added, it tends to decrease the filler dispersibility, so the effect is insufficient, and still could not resolve the present problems.
In an electrophotographic photoconductor which contains a filler to increase durability, in order to realize high image quality, it is important not only that image blurring or residual potential rise is suppressed, but also that charge reaches the photoconductor surface linearly without the filler in the protective layer interfering with charging. This is largely affected by filler dispersibility in the protective layer. If the filler agglomerates, and charge implanted from the charge transport layer to the protective layer moves to the surface, the progress of this charge tends to be obstructed by the filler, the dots formed by the toner become scattered, and resolution considerably decreases. Also, if a protective layer is provided, and the writing light is scattered by the filler so that the optical transmission decreases, there is likewise a marked unfavourable effect on resolution, and this effect on optical transmittance also has a close relationship with the filler dispersibility. The filler dispersibility also has a large impact on wear resistance. When the filler strongly agglomerates and dispersibility is poor, the wear resistance largely decreases. Therefore, in an electrophotographic photoconductor wherein a protective layer containing a filler is formed to improve durability, in order to simultaneously obtain high image quality, it is important not only to suppress image blurring and residual potential rise, but also to enhance the filler dispersibility in the protective layer.
However, an effective method of resolving all these problems had not yet been found, and if the outermost surface layer of the photoconductor were made to contain a filler to improve durability, image blurring or residual potential rise was very marked, and image quality problems had not yet been resolved. To mitigate these effects, it is necessary to install a drum heater, but high durability of small diameter photoconductors for which durability is most important had still not yet been achieved, and this was a major obstacle to achieving compactness and reducing power consumption.